The Era of Classical Spectroscopy

Spectral nature of light

[Picture of Sir Isaac and picture of expt in front of Royal
Society]

Although the spectral nature of light is present in the rainbow,
it was beyond the ability of early man to recognize its significance.
It was not until 1666 that Newton showed that the white light from
the sun could be dispersed into a continuous series of colors. Newton
introduced the word "spectrum" to describe this phenomenon.
His instrument employed a small aperture to define a beam of light,
a lens to collimate it, a glass prism to disperse it, and a screen
to display the resulting spectrum. This first spectroscope was nearly
in modern form. Newton' s analysis of light was the beginning of
the science of spectroscopy.

It gradually became clear that the sun's radiation has components
outside the visible portion of the spectrum. W Herschel (1800) demonstrated
that the sun's radiation extended into the infrared, and J.W. Ritter
(1801) made similar observations in the ultraviolet. These studies
were the precursors of radiometric and photographic measurements
of light, respectively.
Spectral lines and their quantitative measurement

[Picture of Fraunhofer and picture of solar spectrum with
dark lines]

The achievements of Joseph Fraunhofer provided the quantitative
basis for spectroscopy. Fraunhofer, born near Munich in 1787, extended
Newton's discovery by observing that the sun's spectrum, when sufficiently
dispersed, was crossed by a large number of fine dark lines (1814),
now known as Fraunhofer lines. W.H. Wollaston had earlier observed
a few of these lines (1802), but failed to attach any significance
to them. These were the first spectral lines ever observed, and
Fraunhofer employed the most prominent of them as the first standards
for comparing spectral lines obtained using prisms of different
glasses. Fraunhofer also studied spectra of the stars and planets,
using a telescope objective to collect the light. This laid the
foundation for the science of astrophysics.

[Add picture of Thomas Young and interference pattern]

Fraunhofer also developed the diffraction grating, an array of
slits, which disperses light in much the same way, as does a glass
prism, but with important advantages. With a prism, the angle at
which a spectral line is dispersed depends on the type of glass
used. This makes it difficult to compare different spectral measurements,
and absolute wavelength measurement is not possible. Gratings, which
employ interference of light waves to produce diffraction, provide
a means of directly measuring the wavelength of the diffracted beam.
Earlier, T. Young had demonstrated that a light beam passing through
a slit emerges in a pattern of bright and dark fringes. Fraunhofer
extended these studies to the case of two, three and many closely
spaced slits, and thus developed the transmission grating. With
this, he was able to directly measure the wavelengths of spectral
lines. Fraunhofer's achievements are all the more impressive, considering
that he died at the early age of 39.

Atomic identity of line spectra

[Picture of Kirchoff and picture of elemental spectra]

Despite his enormous achievements, Fraunhofer did not understand
the origin of the spectral lines he observed. It was not until 33
years after his death that Kirchhoff established that each element
and compound has its own unique spectrum, and that by studying the
spectrum of an unknown source, one can determine its chemical composition.
With these advancements, spectroscopy became a true scientific discipline.
In the early 1800's many workers, J.F.W. Herschel, W.H.F. Talbot,
C. Wheatstone, A.J. Angstrom, and D. Alter among them, studied spectra
from terrestrial sources such as flames, arcs and sparks. These
sources were found to emit bright spectral lines, which were characteristic
of the chemical elements in the flame. Foucault, the French physicist,
observed in 1848 that a flame containing sodium would absorb the
yellow light emitted by a strong arc placed behind it. This was
the first demonstration of a laboratory absorption spectrum.

These facts were brought together in 1859 by G. Kirchhoff in his
famous law, which states that the emitted power and absorbed power
of light at a given wavelength are the same for all bodies at the
same temperature. It follows that a gas, which radiates a line spectrum
must, at the same temperature, absorb the spectral lines it radiates.
From this, Kirchhoff and R. Bunsen explained that the Fraunhofer
lines in the sun' s spectrum were due to absorption of the continuous
spectrum emitted from the hot interior of the sun by elements at
the cooler surface. Analysis of the sun's atmosphere thus became
possible.

By recognizing that each atom and molecule has its own characteristic
spectrum, Kirchhoff and Bunsen established spectroscopy as a scientific
tool for probing atomic and molecular structure, and founded the
field of spectrochemical analysis for analyzing the composition
of materials. These techniques are used today to analyze both terrestrial
and stellar objects, and it continues to be our only means of studying
the chemical elements present in stars.